The recent WMAP3 results have placed measurements of the spectral index n _ { { S } } in an interesting position .
While parameter estimation techniques indicate that the Harrison–Zel ’ dovich spectrum n _ { { S } } = 1 is strongly excluded ( in the absence of tensor perturbations ) , Bayesian model selection techniques reveal that the case against n _ { { S } } = 1 is not yet conclusive .
In this paper , we forecast the ability of the Planck satellite mission to use Bayesian model selection to convincingly exclude ( or favour ) the Harrison–Zel ’ dovich model .